The present invention relates to a current resonant converter such as a flyback current resonant converter for obtaining a particular DC voltage from another DC power voltage.
Thus far, a flyback current resonant converter has been widely used as a power device for various electronic units.
In such a current resonant converter, with respect to the protection and efficiency of a switching device, it should be turned on and off when a current which flows in a series resonant circuit is zero (hereinafter, this operation is named the "zero current switching").
However, in the conventional current resonant converter, even if the changing ranges of a load current and an input voltage are known, the circuit constants of the current resonant converter are not determined with them. Thus, the zero current switching cannot be always accomplished in the optimum timing.
Consequently, it is necessary to use a switching device and other parts with current ratings which exceed those actually required and thereby increasing the size and cost thereof.
On the other hand, when a frequency modulation control is performed so that the output voltage is stably kept in the full changing ranges of the input voltage and the load current, the sine component of the current which flows in the switching device becomes very small and thereby disabling the turn-off operation by the zero current switching.
In particular, when the load is light, a dummy load should be provided so that the apparent load exceeds a predetermined value even if no load is applied. Thus, the cost performance is remarkably decreased.
As was described above, in the conventional current resonant converter, with the changing ranges of the load current and the input voltage, the optimum operating point of the switching device cannot be determined. Thus, it is difficult to stably operate the stable zero current switching. In addition, it is necessary to use a part with current rating which exceeds that actually required and a dummy load or the like.